It is well known that it is very difficult to form an ohmic contact to a p-type zinc selenide layer. This is particularly so, even for a heavily doped P-type zinc selenide, especially if the dopant is nitrogen.
The cause of this problem is probably due to the relatively high Schottky barriers of the various metals used to make the contact, these barriers being about 1 eV.
There have been many attempts to solve this problem. Thus, Uemoto J. Appl. No. 62-297359 teaches forming an ohmic contact by growing a p-type amorphous Si layer by a photo-assisted CVD method on the p-type ZnSe layer and then depositing Mo and Al on the Si layer. However, since there is no grading of the p-type amorphous layer, a barrier of only a few tenths of a volt may result in the prevention of the formation of an ohmic contact. Additionally, since there is no continuity of the lattice of the ZnSe layer into the deposited amorphous Si layer, the amorphous layer probably can be easily peeled away from the ZnSe layer thus resulting in poor mechanical integrity of the Si-pZnSe contact.
Okawa J. Appl. No. 62 16921 teaches the formation of an ohmic contact by the evaporation of a gold layer directly on the surface of a p-type ZnSe layer to which nitrogen ion acceptors have been added.
The contact achieved is not an ohmic contact. It has been found, however that the contact formed is not an ohmic contact but is a Schottky contact with a barrier of about 1.4 eV. Such a contact requires the application of about 15 V or larger to achieve current density in the order of 10 A/cm.sup.2.
Basol U.S. Pat. No. 4,456,630 teaches a method of forming ohmic contacts on IIB-UIA p-type doped layers by etching the surface of the layers with an acidic solution to form a surface that is non-metal rich, treating the etched surface with a basic solution and then depositing a conductive metal layer. The only example shown of the p-type material is p-type cadmium telluride. Moreover, the method shown here does not work for p-ZnSe.
McCaldin et al U.S. Pat. No. 4,123,295 teaches, in general, that an improved contact may be made on a p-type semiconductor body such as a p-type zinc selenide by forming a layer of a mercury cholcogenide such as MgSe, MgS or HgTe on its surface.
However, in the example shown in this patent, an ohmic contact is made on an n-type zinc selenide body doped with aluminum by wetting the surface of the body with an amalgam of HgInCd and heating to 450.degree. for 1 minute.
In any event there is no suggestion in this patent of forming a ohmic contact on a p-type ZnSe semiconductor body in which the dopant is N. Heating at a temperature of 450.degree. C. as shown in the example of the patent would not be useful as it would destabilize the N-dopants present and would tend to decrease the effective concentrations of the N dopants.
The McCaldin et al patent also shows cleaving of the resultant sample and the formation of an epitaxial layer of stoichiometric HgSe by evaporation on the sample of Hg and Se. The patent indicates that a ohmic contact is not found because it states that "All measurement showed a barrier significantly higher than the Au barrier on the same substrate", col. 2, lines 63-65.
Kamata J. Appl. No. 63-10826 shows the formation of an ohmic contact on a p-doped ZnSSe layer by providing an intermediate layer of p-type GaAs between the metal contact layer and the ZnSSe layer. However, it has been found that here too an ohmic contact is not found since the barrier between the p-type ZnSe and p-type GaAs is about 1 eV.